1. Field of the Invention
The invention relates to an integrated services digital signal transmission system for joint transmission of narrow-band and broadband signals.
2. Description of the Related Art
When structuring an integrated services digital network ISDN, precautions must be taken for co-operation between the speech, text, data and picture communication arrangements for accessing the network and handling of communication. The periodical "Telecom Report", No. 6, (1983), volume 3, pages 166 etc. describes the subscriber access architecture specified in the CCITT-recommendation I. 430. Two communication channels (B-channels) each having 64 kbit/s and a signalling channel (D-channel) having 16 kbit/s are available at the interface between the network and the subscriber, commonly referred to as the SO-interface, for simultaneous two-way transmission.
The subscriber signalling information required for the two B-channels are transmitted via the D-channel. This signalling operation includes the transfer of general information between connected subscriber terminal devices and the network, for example about non-fulfilled requested external connection setting-up, toll ticketing indications, as well as the transfer of information for the service-specific handling of the connections on the B-channels inclusive of the connection set-up control in the case of changing terminal set configurations (subscriber stations in which a plurality of sets are connected to one terminal).
In ISDN, the subscriber stations are connected to a what is commonly referred to as a network termination NT which is connected to a line terminal LT of the exchange via the connecting line. Line termination LT and the network termination NT take over the transmission technical tasks of sending and receiving the narrow-band signals at a net rate of 144 kbit/s, which includes the task of clock pulse supply and synchronization, error limitation by loop formation and remote feeding.
Up to eight terminal sets can be connected in ISDN to the SO-interface, which can access the D-channel independently of each other. In order to assure that while one such subscriber terminal set transmits a message the other subscriber terminal sets are prevented from also starting message transmission, the CCITT recommendation I-430 specifies that first of all the D-bits arriving from the terminal sets at the network termination via the D-channel are individually returned, in addition to their transmission to the exchange, in a special D-echo channel to the terminal sets via the SO-interface. The electrical properties of the SO-interface are chosen such that a logical "0" always dominates when several terminal sets simultaneously send a logic "0" and a logic "1" on the C-channel (wired or). The network termination NT returns a logic "0" via the D-echochannel (E-bit), so that any terminal set which has sent a logic "1" to the network termination NT recognizes this deviation and stops its message before the subsequent D-bits are transmitted.
In a future broadband-ISDN with preferably optical subscriber connecting lines, services operating with higher transmission bit rates (for example moving picture services) are planned, in addition to the ISDN narrow-band services mentioned in the foregoing. Whilst, when existing copper wire lines are used, this integration is limited to services which can manage not more than two 64 kbit/s-B-channels, there will be no limitations as regards the capacity of the connecting lines. For accessing the network at the broadband interface, when a broad-band terminal station is connected, it is recommendable to specify an access method in view of the integration of broadband services on the subscriber connecting line. Should the access to the broadband terminal set be effected using the access method laid down in the CCITT-recommendation I. 430, a possible solution is to connect the broadband terminal set to the network termination via a multiple line selector. For this purpose the broadband terminal set is connected just as a narrow-band terminal set to the SO-bus and also, in parallel therewith, via a broad-band connection (on which only the broadband information is transmitted) to the network termination NT. Such a solution of jointly accessing the D-channel has the disadvantage that an additional line is required between the broad-band terminal set and the SO-bus, so that the requirement of the transmission-technical integration already in the terminal set is not satisfied.
Published European Patent Application No. EP-B1 0,053,236, which corresponds to U.S. Pat. No. 4,545,048, discloses an integrated services digital transmission system in which at the transmitter end a low-rate and a high-rate signal are superimposed in separate frequency transmission ranges. At the receiver end the received sum signal is again split into the low-rate and the high-rate signals by means of frequency filtering. For separation according to frequency of the low-rate and high-rate signals, steep-edged LC filters are required, it being necessary for the frequency spectra of the low-rate and high-rate signals to be adequately spaced apart. Furthermore, the integrated services digital transmission system described in the patent has the disadvantage that, owing to non-linearities, the intermodulation products produced during transmission cannot be eliminated by frequency filtering and consequently the transmission quality is affected.
Furthermore, published European Patent Application No. EP-A1 0,135,164 discloses a transmission system for digital signals having a high modulation rate with additional signal transmission, in which the low-rate signal is superposed, as regards its phase, on the high-rate signal, the clock phase of the high-rate signal being modulated with the low-rate signal. For the transmission of the low-rate signal the high-rate signal must be available--at least as a clock signal--so that limited operation to economise power consumption, more specifically of the narrow-band channel only, when the local mains current supply fails and buffer battery supply is provided for the transmission arrangement at the subscriber side, is not possible.